Conventionally, a technique of speech encoding that performs data compression by encoding speech has been used in various fields such as mobile phones, Voice over Internet Protocol (VoIP), broadcasting, and recording, namely, demand for this technique has been increasing. In some cases, confidentiality of communication is enhanced by encrypting encoded information. In this case, the encryption is first decrypted to retrieve an encoded signal that has been encoded by various speech encoding techniques. Then the encoded signal is decoded into a speech signal for enabling reproduction of speech.
Until decrypting processing is performed on the encoded signal that has been encrypted, if a part of the encoded signal that has been encrypted is lost, or an error occurs due to a noise or a transmission failure, the encryption cannot be decrypted and therefore in some cases, the encoded information cannot normally be decoded. Furthermore, when information about an encryption key contains an error, encoded information after decrypting the encryption also contains an error. Thus, a speech signal to be decoded is considerably damaged. In a case where the encoded information is not normally decoded, when a decoded signal having excessive amplitude is reproduced, an abnormal sound with large volume might be generated. Note that, when the encryption is omitted, an error may directly contaminates the encoded signal on, for example, a transmission path before decoding the encoded signal, and then the above-described case may be caused.
As a technique for avoiding such an error, for example, PTL 1 discloses a mobile device (decoding device) that adds an encryption header to an encoded signal upon encrypting, and removes the encryption header upon decoding to perform decoding based on information about encrypting algorithm used for encoding, which is contained in the encryption header.
PTL 2 discloses an encrypting device and a decrypting device. The encrypting device divides speech code information into encryption target data and non-encryption target data, and encrypts the encryption target data using an initial vector generated based on the non-encryption target data and a predetermined key, and combines those sets of data to transmit to the decrypting device. The decrypting device generates the initial vector by separating the non-encryption target data upon decrypting, and performs decrypting using the predetermined key and those sets of data.
As a technique for avoiding generation of an abnormal sound with large volume caused by an error, for example, PTL 3 discloses a speech suppressing device that stops output of the speech signal, when a decoded speech signal has a higher level than a predetermined threshold.